the version of iftop in your repository as shown is
which says the package is for RHEL 5
and the new package libpcap which is already installed in your system is
which is of RHEL 6
so can you tell us which version of RED HAT are you using and which repository you are using?
as when you do
# yum install iftop
yum should look for RHEL 6 packages if your machine is RHEL6 and not RHEL 5